1. Field of the Invention
The present invention relates to an optical disc having an effect of indicating characters and pictures by making use of light diffraction.
2. Description of the Related Art
There has been known a technique in which diffracted light, caused by pits formed on one surface of an optical disc, can be used to indicate characters and pictures on the surface of the disc. In industry, such a technique is usually referred to as a mastering art or a pit art. In practice, when a mastering art disc is formed by bonding together two single-plate discs, such as in a process in which a DVD is manufactured, the outer surface of one of the two single-plate discs is used as a label-indication surface to indicate characters and pictures, so that the title of the DVD may be expressed thereon.
Further, there have been suggested various methods for manufacturing an optical disc using mastering art. However, in practice, a stamper for forming such an optical disc may be manufactured with the use of a conventional mastering process that includes the following steps and will be described as below. (1) A photoresist is applied on to the surface of a master disc made of a glass so as to form a resist layer thereon. (2) A laser cutting machine is used to produce a laser pulse so as to intermittently irradiate the glass-made master disc to form latent pit images on the resist layer. (3) A developing treatment is carried out and some exposed portions of the resist layer are removed. (4) A conductorizing treatment is carried out to form a conductive film on the surface of the resist layer and on some portions of the master disc from which the resist substance has been removed. In detail, the conductive film may be formed by a metal such as Ni, with the use of a film formation process such as electroless plating, sputtering or vapor deposition. (5) An electrocasting treatment is carried out to form a metal layer such as an Ni layer over the conductive film, the electrocasting treatment being continued until a metal layer having a predetermined thickness is formed. (6) The metal layer is removed from the glass-made master disc. (7) The back surface of the metal layer is polished and is then subjected to a pressing treatment so as to form a predetermined outer diameter and a predetermined inner diameter.
Here, the above step (2) will be described in more detail with reference to FIGS. 1 and 2. For example, as shown in FIG. 1, when a character xe2x80x9cKxe2x80x9d represented by a pattern 103 is indicated in the recording area of an optical disc stamper, pits are not formed in an area 102 of the pattern 103, but are formed in an area 101 excluding the area 102. FIG. 2 is an enlarged view schematically indicating the area 101. In the step (2), the glass-made master disc is rotated, and the laser beam is linearly moved in the radial direction of the disc in a manner such that the area 102 is not exposed while the area 101 is exposed by an intermittently produced pulse. In this way, a stamper made in this step is formed with a spiral track 202 with the rotation center 104 serving as its spiral center, thereby forming many pits 201 at a predetermined interval along the track 202.
Further, when an optical disc is manufactured based on the stamper, it can use the same method as used for manufacturing a common optical disc. Namely, after a resin such as polycarbonate is injection molded, a sputterring apparatus is used to form a reflecting film such as an AlTi film on one surface of the disc having a plurality of pits formed thereon.
FIG. 3 is a view schematically indicating a cross section of an optical disc in its circumferential direction (direction xcex8 in FIG. 1). As shown in FIG. 3, a plurality of pits 305 formed on a stamper at a predetermined interval have been transferred on to one surface of a polycarbonate substrate plate 301. A reflecting film 302 is formed on that same surface of the substrate plate. Here, a center-to-center interval between every two pits is set to be P. Light rays 303 incident on the substrate plate 301 have various spectral characteristics due to different environments. However, the light ray used here is set to be a white light containing equivalently all the light components having various different wavelengths in the range of the visible light. Here, if P is set to be about 1 xcexcm, the light ray 303 passing through the substrate plate 301 will be reflected by the reflecting film 302, thereby obtaining a reflected light. At this time, only some light rays of the reflected light having a specific wavelength in a specific direction will be strengthened by each other, so that these light rays become diffracted light 304 and are allowed to emit outwardly through the substrate plate 301.
FIG. 4 is an explanatory view schematically showing a positional relationship between the diffracted light (represented by reference numeral 304 shown in FIG. 3) from the optical disc 401 on one hand and the position of a viewer on the other. As shown in the figure, the head of an observer 402 is located in the normal line direction of an optical disc 401, so that the observer can observe the optical disc 401 from an upper position above the disc. Here, the observer 402 is assumed to be in a typical living environment, and the light ray 303 is assumed to be a white light ray. Further, the diffracting angles of the diffracted light rays 304 to be observed will be different from one another depending upon their different positions on the optical disc. Further, since the light rays 303 may also be considered to have various different incident angles, various diffracted light rays 304 having different wavelengths can be observed as if a rainbow-like image is formed on the surface of the optical disc.
In this way, by virtue of the above-described diffraction, within the surface of the optical disc 401, the area 101 having pits formed thereon can be observed to -have bright rainbow colors. On the other hand, the area 102 having no pits formed thereon will be observed to have a dark color since there is no diffraction occurring thereon. By virtue of such a contrast, a character like xe2x80x9cKxe2x80x9d shown in FIG. 1 will become clearly visible.
In order to clearly indicate pictures or characters formed on an optical disc, it is desirable that the diffracted light rays in the area where the pits are formed be bright enough to be clearly observed. In the above example, the wavelength of each diffracted light 304 and the light amount thereof will depend upon the interval between every two pits and pit shape. Further, since people will have different perceptions of different wavelengths of visible lights rays, and since it is desired to clearly observe the diffracted light rays, a suitable pit interval and pit shape is required.
An object of the present invention is to solve the above described problems and a specific object is to calculate the best pit interval and best pit shape that are effective for obtaining a maximum light amount of diffracted light rays, thereby making it possible to clearly indicate any pictures or characters formed on a mastering art disc.
According to the present invention, it is possible to provide the following first to third types of optical discs.
A first type optical disc according to the present invention has a circular disclike substrate including areas on which a plurality of pits or grooves have been formed, such disc having an effect of indicating characters and pictures by making use of a property in which incident light irradiating the pits or grooves will be diffracted, characterized in that: an interval Pr (nm) between every two pits or every two grooves in the radial direction of the optical disc is 570xe2x89xa6Prxe2x89xa61000.
A second type optical disc according to the present invention has a circular disc-like substrate including areas on which a plurality of pits or grooves have been formed, such disc having an effect of indicating characters and pictures by making use of a property in which incident light irradiating the pits or grooves will be diffracted, characterized in that: an interval Pxcex8 between every two pits or every two grooves in the circumferential direction of the optical disc is 570 nmxe2x89xa6Pxcex8xe2x89xa61000 nm.
A third type optical disc according to the present invention has a circular disc-like substrate including areas on which a plurality of pits or grooves have been formed, such disc having an effect of indicating characters and pictures by making use of a property in which incident light irradiating the pits or grooves will be diffracted, characterized in that: when the depth of each pit or each groove is d and the refraction index of the substrate is n, nd satisfies 90 nmxe2x89xa6ndxe2x89xa6200 nm.
However, each of the optical discs made according to the present invention is formed by bonding together two single-plate discs. Preferably, specifically designed concave/convex structure (depending upon an interval between every two pits or every two grooves, as well as the depth of each pit or each groove) is formed on the surface of the substrate of one single-plate disc having only one surface as its recording surface, thereby obtaining a desired mastering art effect by virtue of the areas having the concave and convex structure.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.